The factors influencing the pathogenesis of autoimmune disease are not fully known, but the host genotype undoubtedly plays a role in determining the outcome of these diseases. The role of the host's major histocompatibility complex (MHC) genotype in the regulation of susceptibility to autoimmune diseases has been extensively studied in different populations, and certain HLA (the human MHC) alleles and haplotypes have been reported to be associated with several autoimmune diseases. In particular, the association with genes from the HLA-B8,DR3 haplotype has been reported by different research groups. This haplotype is associated in all Caucasian populations with a wide variety of diseases with autoimmune features, and in healthy subjects it is associated with a number of immune system dysfunctions. Mainly, peripheral blood mononuclear cells from HLA-B8,DR3-positive and -negative individuals differ in their ability to produce interleukin (IL)-2, IL-5, IL-12 and interferon-gamma upon stimulation with the mitogen phytohaemoagglutinin (PHA), while producing similar amounts of IL4, IL-6 and IL-10. Furthermore, in HLA-B8,DR3-positive subjects tumor necrosis factor alpha secretion is increased both with and without PHA stimulation. Accurate control of the functional repertoire of an immune response is a critical parameter in the response to infections as well as in immunopathology. MHC control of the class of the immune response at the level of cytokine production is a sophisticated way in which this occurs. This control might be involved in adaptive immune responses to infections as well as in immunopathology.